update drgn type instantiation routines

[ahrens]

drgn changed the way what you instantiate a type object, so we need to
update type_canonicalize().

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build was run locally and any changes were pushed
• Lint has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

  File "/usr/lib/python3/dist-packages/sdb/command.py", line 730, in caller
    method.input_typename_handled)] = method
  File "/usr/lib/python3/dist-packages/sdb/target.py", line 128, in type_canonicalize_name
    return type_canonical_name(prog.type(type_name))
  File "/usr/lib/python3/dist-packages/sdb/target.py", line 120, in type_canonical_name
    return str(type_canonicalize(t).type_name())
  File "/usr/lib/python3/dist-packages/sdb/target.py", line 110, in type_canonicalize
    return drgn.pointer_type(t.size, type_canonicalize(t.type))
AttributeError: module 'drgn' has no attribute 'pointer_type'

What is the new behavior?

dbufs, spa -v, etc work.

Does this introduce a breaking change?

• Yes
• No

Other information

[codecov-commenter]

Codecov Report

Merging #239 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #239   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          60       60           
  Lines        2473     2473           
=======================================
  Hits         2161     2161           
  Misses        312      312           

Impacted Files	Coverage Δ
sdb/target.py	97.61% <100.00%> (ø)

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